1. Field of the Invention
The present invention relates to a manufacturing method and manufacturing apparatus of a screw-threaded bottle-can having a screw-thread on which a cap is screwed.
2. Description of the Related Art
A can (a bottle-can) having a bottle-shape of aluminum alloy in which a cap is screwed on a mouth section having a screw-thread is known as a container filled with contents such as drinks.
The bottle-can is, as disclosed by U.S. Pat. No. 5,704,240, Japanese Unexamined Patent Application, First Publication No. H05-229545, and Japanese Unexamined Patent Application, First Publication No. 2002-66674, manufactured by: performing a drawing processing and an ironing processing (i.e., a DI forming) so as to form an aluminum alloy sheet into a cylindrical body having a bottom plate part and a cylindrical-lateral part being integral with each other; narrowing an aperture section so as to form a shoulder part; forming a neck part of a small diameter on an upper end of the shoulder part and forming an expanded pipe part on an upper part of the neck part for screw-forming; performing a screw-forming processing on the pipe part; and performing a curl-forming processing on an aperture-end part, and the like.
In the manufacturing process of the bottle-can, a screw-thread part is formed by inserting the pipe part between an inner mold inserted in the pipe part and an outer mold outside the pipe part, and revolving the inner mold and the outer mold around an axis of the can while rotating the inner mold and the outer mold on their own axes. The screw-thread part is usually a right-hand screw. In order to form the right-hand screw, a right-hand screw-type protrusion is formed on the inner mold, and a left-hand screw-type protrusion is formed on the outer mold.
Japanese Unexamined Patent Application, First Publication No. 2002-66674 describes a method of processing an aperture part without damaging to an inner coat of a can, by forming a mouth section to have at least two or more steps from a shoulder part in an intermediate-formed product before a screw-forming processing by a drawing processing; a pipe part is formed on a second step from the aperture end to have an outer diameter of an intermediate diameter between an outer diameter of a screw-ridge and an outer diameter of a screw-bottom; and a screw-forming process is performed by inserting the pipe part between an inner mold and an outer mold.
The present applicant has proposed in Japanese Unexamined Patent Application, First Publication No. 2004-74168 to reduce torque for closing a cap again after once opening, by setting a height of a first step of a screw-ridge from an aperture end to be lower than heights of second and subsequent steps of the screw-ridge among a plurality of steps of the screw-ridge.